Television apparatus



Feb. 21, 1939. AA w. vANcE ET AL TELEVISION APPARATUS Filed May 24, 1934 ze figg 3ra/mm,

WMM@ #77705' K Patented Feb. 2l, 1939 PATENT OFFICE TELEVISION APPARATUS Arthur W. Vance and Harry Branson, Philadelphia, Pa., assignors to Radio Corporation o! America, a corporation of Delaware Application May 24, 1934, Serial No. 721,228

5 Claims. (Cl. 11S-1.2)

Our invention relates totelevision apparatus and particularly to television transmitter tubes of the cathode-ray type In television transmitter tubes of the type de- 5 scribed in an article by Dr. V. K. Zworykin entitled Televislon which appears in the January, 1934, issue of the Journal of the Franklin Institute, an image of the picture to be transmitted is formed uponva mosaic of minute light-sensitive elements supported on and insulated from a common metallic back-plate, each element being the cathode of a photo-electric cell. Each of these photo-electric cell cathodes acquires a positive potential having a value which depends upon the 15 intensity of the nght striking it. As wm be more fully explained hereinafter, the mosaic is scanned by a cathode-ray beam whereby picture current appears in an output circuit, the picture current having a value depending upon said positive potentials.

'I'he main object of our invention is to improve the sensitivity and/or eiliciency of a cathode-ray transmitter tube of the above-mentioned type.

More specifically, an object of our invention is 25 to apply a negative polarizing potential to the cathode of a photo-electric cell unit in atrans mitter tube of the above-mentioned type.

In practicing our invention, we provide the transmitter tube with an auxiliary source of electrons and spray these electrons upon the lightsensitive mosaic. The electrons are projected towards the mosaic at a comparatively low velocitywhereby a negative potential is built up on 35 the photo-electric cell elements or cathodes to produce an increased potential difference or polarizlng voltage between the cathode and anode of each photo-electric cell. In one embodiment of our invention the electrons are sprayed upon the 40 mosaic continuously, while in another embodiment the electrons are sprayed thereon only during an interruption in the scanning, as at the end of each horizontal scanning line.

'I'he invention will be more clearly understood from the following description read in connection with the accompanying drawing in which Fig. 1 is a view of a television transmitter tube and associated circuit constructed in accordance with one embodiment of our invention;

Fig. 2 is a view of another embodiment of our invention;

Fig. 3 is a schematic diagram of the photo-electric cell circuit of the tube shown in Figs. 1 and 2;

and

Fig. 4 is a set of curves which are referred to in explaining one embodiment of the invention.

Referring to Fig, 1, our invention is shown applied to a television transmitter tube of the character described in the above-mentioned publica- 5 tion. The tube comprises a highly evacuated envelope I in which is located an electron gun consisting of a cathode 3, a control electrode 5 and a first anode 1. The anode 1 is maintained at a suitable positive potential with respect to the cathode by means of a battery 9 while the control electrode is maintained at a suitable negative potentlalby means of a battery II.

The electrons leaving the rst anode 1 are accelerated, and the electron beam focused to the desired small cross-sectional area, by means of a second anode I3 which consists of a metallic coating on the inner surface of the envelope I. 'I'he second anode I3 is connected to ground and maintained at a comparatively high positive potential by means of a battery I5 connected in series with the battery 9.

A mosaic I1 of light sensitive elements is supported in the envelope I in such position that it may be scanned by the electron beam. It is also positioned to permit an image of an object to be transmitted to be formed thereon by means of a suitable lens system I9. Deflecting devices such as deecting coils 2I and 23 are provided for deflecting the electron beam in a horizontal plane and in a vertical plane, respectively.

Picture signals from the transmitter tube are impressed upon an amplifier 25 through an output resistor 21 which is connected between backplate of the mosaic I1 and ground. Preferably, 35 a biasing battery 29 is included in the circuit between they resistor 21 and ground for applying a negative bias to the control grid of the amplifier tube 25. I

'I'he mosaic I1, which in itself is no part of the 40 present invention, may be constructed in a variety of ways. In a preferred form it consists of a thin sheet of mica having a continuous metallic coatlng on the back side thereof which is electrically connected to the output resistor 21. This metallic coating will be referred to as the signal plate. 'Ihe mosaic proper may consist of a multitude of silver globules each of `which has a nlm of silver oxide thereon, the silver oxide lm being coated with caesium. In such a mosaic, the light sensitive elements are electrically insulated from each other and from the metallic coating or signal plate on the back of the mica sheet. Another possible method of constructing a mosaic is that side of the mica and separating the light-sensitive material into a large number of elements insulatedfrom each other by ruling the light sensitive/fum with a ruung machine. stili anothermethod-of vmaking a mosaic is to form an exceedinglythin coating of light-sensitivev material upon the mica, the coating not necessarily being discontinuous, but being so thin that the resistance between picture elements is very high,

Each light-sensitive element of the mosaic I1 acts as the cathode of a photo-electric cell which has the metallic coating I3 as its anode, the second anode I3 functioning not only as an accelerating and focusing electrode for the electron beam but also as a common photo-electric cell anode for `the photo-electric cell cathodes in the mosaic.

The photo-electric cell circuit of the transmitter tube is shown in Fig. 3 where parts similar to those in Fig. l areindicated by the same reference numerals. Two light-sensitive globules or cathode elements of the mosaic I1 are indicated at 3| and 33.- 'Ihe common anode I3 for the cathodes 3| and 33 is connected through the biasing battery 29 output resistor 21 to the metallic coating indicated at 35 which is on the back side of the mica sheet (not shown). photo-electric cell cathodes 3| and 33 are connected to the metallic coating or signal plate 35 only through the capacity between the silver` globules and the said metallic coating. These capacity connections are represented by the condensers 31 and 39.

The operation of the transmitter tube depends upon the fact that when an image is thrown upon the mosaic I1, each light sensitive element or globule acquires a positive charge having a value depending upon the intensity of the light striking it. 'I'he positive charge, of course, is acquired as a result of the light sensitive element emitting electrons. When a light sensitive element acquires a positive charge, the condenser which it forms with the signal plate is likewise charged so that each time a light sensitive element is scanned by the electron beam, the condenser is discharged through the output resistor 21. This discharge current, which has a value depending upon the intensity of light at a certain picture area, is the picture signal current.

As is well known, the scanning beam brings the mosaic to a certain equilibrium potential each time it scans the mosaic. Thus, at the end of a scanning period, the mosaic is in condition to repeat the cycle of operation.

It will be apparent from an inspection of Fig. 3 that if the photo-electric cell anode I3 could be made positive with respect to the photo-electric cell cathodes 3| and 33, the said cathodes would acquire a greater positive charge than otherwise when a picture image is thrown upon the mosaic. In other words, if the photo-electric cells 3I-I3 and 33-I3 could be given a polarizing potential such that the photo-electrons which are freed from the cathodes 3| and 33 would be pulled over to the common anode I3, the sensitivity of the transmitter tube would be increased. Since there is no conductive connection to the individual light sensitive elements, this cannot be done by inserting a battery in' the circuit in accordance with usual practice.

In accordance with our invention, we make the light sensitive elements negative with respect to the common anode by spraying electrons thereon at a comparatively low velocity.

Referring again to Fig. 1, the electrons for electrons striking the mosaic.

It will be seen that the of forming a light sensitive layer upon the front,

spraying the mosaic f I1 yare supplied from. an auxiliary cathode" which has associated therewith a control yelectrode 43-and van anode 45. Although it is not essential, the anode 45 may consist of a metallic coating on the inner surface of the envelope, this coating being a continuation of the coating I3 which forms the second anode oi the electron gun. It lwill be understood that this entire metallic coating I3 .which forms4 the two anodes functionsas the photo-electric cell anode for the mosaic I1.

The cathode 4I and the control electrode 43 are connected through adjustable taps to a potentiometer 41 connected across the battery I5. It will be noted that no attempt is made to direct the electrons leaving the auxiliary cathode 4| into the form of a beam, but rather, they are sprayed out from the cathode 4| to cover the mosaic formly.

The voltage between the auxiliary cathode 4| and the anode 45 is so adjusted that the electrons reach the mosaic I1 at a velocity which is low compared with the velocity of the electron beam The velocity of the electrons sprayed upon the mosaic should be such that the flow of secondary electrons produced by the bombardment of the sprayed electrons is less than the iiow of electrons striking the mosaic. In other Words, the ratio of secondary electrons to primary electrons should be less than unity. Stated in a different way, the electron velocity should be so low that the electrons will build up a negative charge on the mosaic. The electron velocity which will give this result depends somewhat upon the character of the mosaic and especially upon the nature of the light-sensitive material.

In the case of the above-mentioned mosaic constructed of `the silver oxide and caesium globules, it has been found that the velocity of the electrons at the time they reach the mosaic should be less than 65 volts. In practice it was found that a velocity of 45 volts at the mosaic gave satisfactory results.

Instead of applying the polarizing potential to the photo-electric cells by spraying the electrons upon the mosaic I1 continuously, it may be preferred, in some cases, to spray them upon the mosaic only during the return period of a scanning line or during the period` between picture frames. In Fig. 2, in which parts similar to those in Fig. 1 are indicated by the same reference numerals, there is illustrated a circuit which causes the mosaic to be sprayed by electrons only during the return period of ascanning line.

Referring to Fig. 2, it will be seen that the control electrode 43 is connected through a portion of a potentiometer 49 and through a biasing battery 5| to a point on the potentiometer 41. The potentiometer 49 is connected across the secondary Winding of a transformer 53, the primary Winding of the transformer having one end connected to ground and the other end connected through a coupling condenser 55 to one terminal of a horizontal deflecting coil 2|.

The voltage impulses, which are applied through a coupling condenser 51 to the horizontal deflecting coils 2| for producing a saw-tooth current wave therethrough, are also applied to the primary winding of the transformer 53 so that each time the deflecting coils 2| receive a positive voltage impulse, the control grid 43 also receives a positive voltage impulse.

'I'he biasing battery 5| applies a negative voltage to the control grid 43 of sufficient value to I1 uniblock the ow of electrons from the auxiliary cathode 4|. While the mosaic is being scanned, therefore, it is not being sprayed by electrons. Since the transformer windings are so connected, however, that each time a synchronizing impulse is impressed upon the deecting coils 2|, a positive voltage impulse is applied through the potentiometer 49 to the control grid 43, the negative bias thereon is so reduced that electrons are sprayed upon the mosaic during the duration of the impulse.

The above described action will be more clearly understood by referring to Fig. 4 in which the curve 59 represents the synchronizing voltage impulses which are applied through the coupling condenser 51 to the deilecting coils 2l and to the transformer 53, and in which the curve 6I represents the resulting saw-tooth Wave of current which flows through the defiecting coils. It will be noted that the time relation between the voltage impulses 59 and the current flow through the defiecting coils 2| is such that the return of the electron beam at the end of a scanning line occurs while a voltage impulse 59 is being impressed upon the deflecting coils and the control grid 43. At the end of this return period, the electron beam is again deiiected to scan one line of the picture and during this period no electrons from the auxiliary cathode 4| reach the mosaic I'l.

From the foregoing description it will be apparent that various other modifications may be made in our invention and we desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

We claim as our invention:

1. The method of operating a cathode ray tube of the highly evacuated type having a mosaic of light sensitive elements electrically insulated from each other and from a supporting surface and having an anode common to said elements, which method comprises scanning said mosaic with a beam of high velocity electrons, and spraying said mosaic with low velocity electrons, said low velocity being such that the ratio of secondaryA electrons released from said elements to primary electrons is less than unity whereby said elements assume a negative potential with respect to said anode.

2. The method of operating a cathode ray tube of the highly evacuated type having a mosaic of light sensitive elements electrically insulated from each other and from a supporting surface and having an anode common to said elements, which method comprises producing a beam of comparaless than 65 volts, said low velocity being such that secondary electrons are released from said mosaic with the ratio of secondary electrons to primary electrons less than unity.

3. The method of operatinga cathode ray tube of the highly evacuated type having a mosaic of electron emissive elements mounted upon a supporting surface and insulated from each other and said surface, which method comprises generating a comparatively high velocity electron beam, delecting said electron beam to cause it to scan said mosaic, and spraying said mosaic with electrons continuously during said scanning, said last named electrons having a velocity at said mosaic which is lower than the velocity of the electrons in said beam and which is so low that the ratio of secondary electrons released from said mosaic to primary electrons is less than unity.

4. The method of operating a'cathode ray tube of the highly evacuated type having a mosaic of electron emissive elements mounted upon a supporting surface and electrically insulated from each other and from said surface, which method comprises generating a comparatively high velocity electron beam, scanning said mosaic by said beam, and spraying said mosaic with comparatively low velocity electrons at the end of a scanning movement of said beam whereby secondary electrons are released therefrom, said comparatively low velocity being such that the ratio of said secondary electrons to primary electrons is less than unity.

5. The method of operating a cathode ray tube of the highly evacuated type having a mosaic of light sensitive elements so positioned that it may be scanned on the front side by an electron beam and also so positioned that an image of the picture to be transmitted may be formed on said front side, which method comprises producing a beam of high velocity electrons and scanning said mosaic iwith said beam whereby said mosaic is brought to an equilibrium potential due to the emission of secondary electrons, and spraying said light sensitive elements with electrons having a low velocity as compared with the velocity of the electrons in said beam, said low velocity being of such magnitudethat the ratio of secondary electrons to primary electrons is less than unity.

ARTHUR W. VANCE. HARRY` BRANSON. 

